Multiplex signaling system



J. 5. JAMMER 1,709,076

MULTIPLEX SIGNA LING SYSTEII April 1 6,v 1929.

Filed Sept; '24. 1924 5 Sheets-Sheet Z 1 E E E E E E E E lave/27m Jacob fiJammer by Wm 7 April 16, 1929. J. s. JAMMER MULTIPLE): SIGNALING SYSTEM Filed Sept. 24, 1924 3 Sheets-Sheet 2 mum com mQm lnvenfor Jacob SJammer by Any J. 8. JAMMER MULTIPLEX SIGNALING SYSTEM April 16, 1929.

Filed Sept. 24, 1924 s Sheets-Sheet u ZQFEW hut Mom

ZOE/Eh lnren/or: Jacab fiJam/ner ky my)? Patented Apr. 16, 1929.

UNITED STATES 1,109,07 PATENT OFFICE.

JACOB S. -TAMMER, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC OOMPANY,

INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK. I

MULTIPLEX SIGNALING sys'rnm.

I Application filed September 24, 1924. Serial No. 739,524.

This invention relates to multiplex signaling systems employing a plurality of currents or waves of different frequencies for the transmission of signals or for similar purposes.

An object of the invention is to improve the efiiciency and to reduce the installation and maintenance costs of a multi-channel carrier current signaling system. v

A related object of the invention is to reduce the number of signaling channels which may be lost to service due to the failure of the transmission medium of a carrier system.

The invention comprises a system in which multiplex transmission is applied to a circuit of the four-wire type that is a circuit in which one transmission line is used for transmission in one direction and another transmission line is used for transmission in the opposite direction.

Under certain conditions systems ofthis type are much more eiiicient than two-wire carrier systems. For example, when two systems comprising a totalot say eight signaling channels are operated on a two-wire basis on adjacent lines serious inter-system crosstalk results unless thelines are specially transposed at considerable. expense. But when these eight channels are operated on a physical tour-wire basis in the manner hereina-fter described no line transposition is required other than that necessary to prevent.

singing at repeater points.

Heretot'ore, every four-wire system has g been open to the ()b]6Ct10Il that in case one pair of line conductors should fail the entire system would be put out of service, since it would be useless to carry on a one-way conversation over the line still in service, thus interrupting twice as many carrier channels as in the case of two-wire operation.

This ditficulty is overcome in this invention by providing; means for manually or automatically converting a four-wire system into two two-wire systems in case one of the transmission lines of the four-wire system should fail. In this way half of the transmitting and receiving channels may be operated as a two-wire system over the line that is still intact so that only half of the channels are lost to service.

The circuit for manually converting the four-wire system is so arranged that the switching operations may be-performed by an ordinary plug and jack without reartems.

converting the four-wire system into two two-wlre systems Whenever either transmis-' sion line goes out of service. As long as either transmission line is out of service the entire system is, in effect, set up to operate as two two-Wire systems one of which, of

course, is useless since its transmission line is disrupted, andthe other of which functions 111 a usual way for transmission'in both dilOCtlOIlS; When the disrupted line is cut back in servlce the system is automatically returned to its original condition to function on a four-wire basis. This control is entirely automatlc and requires no maintenance.

The inventlon will be descrlbed as applied to a multiplex carrier current telephone system, although it will be understood that it may also be applied to other signaling sys- In the drawings: a

Fig. 1 is a diagrammatic illustration of a four-wire carrier current telephone system constructed in accordance with the invention.

Fig. 2 is a diagrammatic illustration showingthe system ofFig. 1 converted into two two-wire systems by means of manually 0perated switching devices.

Fig. 3 is a diagrammatic illustration of a, four-wire carrier current telephone system embodying the automatic control feature of j the invention.

The system of Fig. 1 is represented as comprising two terminal stations, W and E, respectively, interconnected by multiplex transmission lineML and ML.

One or more repeater stations R may be connected to the line ML intermediate the terminal stations, similar repeater stations R being connected to the line ML as required. Y

Transmission lines ML and ML each comprise one pair of wires of -a four-wire system which is utilized for the transmission of carrier currents between the two terminals. The l1ne ML 1s used for transmis' line ML is used for transmission from station E'to station W.

A plurality of transmitting channels TO1, TC2, TC-3. etc., at stationW are arranged for simultaneous and lndependent communication over the line ML with cooperating receiving channels RC'1, RC

2, RC3, etc.,' at station E. While eight one-way channels are shown associated with the line ML it will be obvious that the numer may-varyas required.

The transmitting channels TC-1, TC-2,

- TC-3, etc., comprise the usual oscillator,

modulator and transmitting band filter, while the receiving channels BC-I, HG -2, 110- 3 etc.-, comprise the usual receiving -band filter, demodulator and amplifier.

Transmitting and receiving channels of this type are illustrated in Fig. 42 of a paper entitled Carrier telephony and telegraphy by Messrs. Colpitts and Blackwell, published in the Transactions of the American- Institute of Electrical Engineers, volume 40,L1921. fil LP d ow pass grouplng ters V are paire with high pass grouping filters HP at each of. the terminal stations for the purpose of separating the transmitting and receiving channels into upper and lower frequency groups. These filters and each of the other filters shown throughout the system may be constructed in accordance with the principles set forth in the United States patent to Campbell No. 1,227,113, issued May 22, 1917. Filters LP and HP at station W thus separate the lower group of transmitting channels TC 1 to TC-4 from the upper group of transmitting channels TC'5 to TC8 whilelfilters LP and HP at station E separate the lower group of f receiving channels RC1 to RG- i from the upper group of receiving channels RC-5 t0 I Transmitting amplifiers TA 1 and TA- 2 are inserted between the grouping filters.

- LP and HP and the lower and upper groups of transmitting channels, respectively, at

station W. Similar receiving amplifiers RA-11-and RA2 are employed at station E: between the respective grouping filters receiving be of the well known unilateral thermionic electron discharge type.

The repeater B may be of'any well known type such as that disclosed in "the United States patent to Raibourn, No.1,413,357,

issued April 18, 1922. Separate repeater channels are provided for the upper and lower groups of frequencies transmitted in the same direction, thus reducing the load on the'repeater'amplifiers and consequently the likelihood of distortion. Tlris provision also permits one of the repeater channels to reference characters similar to may be associated with channels be utilized for transmission in the opposite direction as is hereinafter more fully explained. The channel fdr repeating the lowep group of frequencies includes a low pass input filter LIF, a unilateral amplilier A1. and a low pass output filter LOF, while the channel for repeating the upper group of frequencies includes a high pass input filter HIF, a unilateral amplifier A2 and a high pass output filter I-IOF.

As is. customary in telephone practice,

the line ML terminates at stations \V and E are also connected to the line ML by means of similar plug and jack connections which, however, have been omitted from the drawmethod of connecting the various circuit elements together is employed at the repeater R.

The various elements of the transmission lines LHJ and ML,, which together comthe upper group ing for the sake of clearness. A similar I prises the four-wire system, are identical.

and hence a description of the line ML is thought unnecessary. The elements associated with the line ML are, designated by those applied to the line ML, but with primes or subscripts afiixed thereto. The direction of transmission of this line is, however, reversed, the transmitting channels of line ML being located at stationE and ,the receiving channels being located at station W.

Each of the transmit-ting channels of the line NEL is associated at station W with a receiving channel of the line ML through the usual hybrid coil circuits, not shown, to form groups hf two-way communication channels. Each transmitting channel of the lower group is preferably associated with a receiving channel of the lower group. Thus, channel TG1 and RC,1, TG-2, RC -2,

-TC3 and RC,3 and so on, are connected res ectively as pairs to low frequency sig' nahng lines which may lead to switchboar or to ordinary subscribers telephone stations. A similar arrangement is employed at station E.

A transmission system employing terminal circuits of this type is disclosed in the United States patent to Espenschied, No. 1,500,552, issued July 8, 1924. The low frequency lines TC-l and RO,1, TC.2 and RO -1, TC3 and Rik-3, TO-5 and BO -5, etc., in the same subscribers at station W and station E, a

subscriber at station IV talking, for example, over a circuit which includes channel TC-l, line ML and channel RC-l, and the subscriber at station E talking over a circuit which includes channel TC 1, line ML, and channel RC,1. In all, eight two way conversations may be carried on simultaneously and independently over the system,

It is well that in transmission circuits, such as telephone circuits, where the lines parallel each other, or by reason of being carried along the same pole line or by reason of being included in the same cable, the telephonic or other signals transmitted over one linewill induce corresponding currents in adjacent lines, thereby producing what is known as crosstalk. introduction in telephone circuits of what are known as line'transpositions at certain intervals for the purpose of balancing out the induced crosstalk currents.

In the case of multiplex carrier current telephone systems the problem of reducing crosstalk becomes one of much greater magnitude than in the case of ordinary telephonic transmission. This follows because the currents induced are greater in the case of the higher frequencies involved, and because the accurate spacing of the transpositions and of the line wires is much more important due to the large number of transpositions required.

In attempting to reduce, crosstalk between carrier current channels: of adjacent lines by means of line transpositions, it is necessary that a large number of transpositions occur per wave length of the highest frequency involved. To effectively transpose the transmission lines involves considerable expense in the case of a carrier transmission circuit in which the wave lengths employed are very much shorter than those employed in ordinary telephone practice, since the transpositions must occur at frequent intervals.

To overcome this difficulty the same group f of frequencies are employed .for transmission til) in opposite directions in the four-wire system of this invention. In other words, the re spective carrier frequencies employed by the transmitting channels TC1, TC2, TC3, etc., at station W' are identical with those employed by transmitting channels TC '1, TC -EZ, TC,3, etc., at station E. If, under these conditions, a voice-modulated wave of 5000 evcles frequency is transmitted over the line ML from transmitting channel TC 1, a corresponding current which might be induced in the line ML, would not produce This has led to the.

wvould be put out of service. ML slu-iuld be disrupted for example, the

interference in the transmitting channels as sociated with line ML at station E, since the one-way terminal amplifiers would not pass currents transmitted in that direction. At

station WV the current induced in line ML,

would be elt'ectivel y prevented from entering all of the receiving channels except channel RC -1 by the high pass filter II'P and the band filters associated with the respective lower group of receiving channels. Such a current would, therefore, only be passed through receiving channel 1iC,1, from which it would be transmitted to the low M line ML would likewise be rendered useless under the old method of operation since there would be no way of transmitting messages from station IV to station E.

This difliculty is overcome in accordance with the invention by a simple switching arrangement at the terminal and repeater stations whereby the system may still function as a two-way'system over the line that is still in service. This feature of the invention will appear more clearly from the consideration of Fig. 2.

IVhen trouble occurs on either of the,

transmission lines ,ML or ML, the attendants at the terminal stations may not know atv once which line is affected, and hence in practice it is preferable to set the foun wire system up as two two- \vire systems, rather than to prepare only one line to operate on a two vire basis. This is accomplished by simply disconnecting one group of transmitting and receiving channels at each terminal station from their respective transmission lines and connecting them to the other transmission line. The corresponding repeater channel also reversed by a similar manual switching operation;

Assuming that one of the transmission lines, which may be the line ML, has been disrupted, and it is desired to utilize the line ML as a two-way transn'lission circuit until the line ML has been repaired, the attendant. at station W will remove patching cords 7 and 7 and the attendant at station E will remove cords 10 and 10 shown in Fig. 1. The line ML is then connected with the uplUU per group of receiving channels at station W by means of patching cord 11 having plugs on either end which cooperate with line ack 6 and station jack 5. The upper group of transmitting channels at station \V are connected to the line ML by means of theeord 12, the conmicting plugs of which cooperate with station jack 5 and line jack (3'. At station E a similar switching operation is made by connecting cord 13 between station jack' 8 and line jack 9' to connect theuppcr group of receiving channels-to the line ML and by plugging the patching cord 1-l into station jack 8 and line jack 9, to connect the upper group of transmitting channels to the line ML. 4 a

Since in each case the terminal connections of the upper group of transmitting and receiving channels have been reversed, the direction of the upper repeating channel at repeaters R and R, must also be reversed if waves are to be transmitted through eitherrcpeater in both directions. T he patching cord connecting filters HIF and HOF to the input and output, respectively, of amplifier A2, shown in Fig. 1, are therefore removed and by means of the same or similar cords filter HIF is connected to the output of'amplifier A-2 and filter HOF is connected to the input of amplifier A2. A similar change is made in the circuit connecting filters HIF, and HOF to amplifier A,2 of repeater R as shown in Fig. 2.

The arrangement of the low frequency lines at the terminal stations must also be changed tocorrespond to the rearranged high freqency circuits. Any suitable switching means, such as plugs and jacks, may be employed for this purpose. When the high frequency circuits has been reversed in the manner described above, transmitting channel T C5 and recelving channel .RC .1 at station W are connected as a pair to a common low frequency line.

Channels TC-(i and RC,-2, TC7 and RO -3 and so on are also connected as pairs to low frequency lines. At station 'E channels RC5 and TC 1, and-so on are similarly connected in pairs to the low frequency lines. For the sake, of clearness, however, the low frequency switching plugs and jacks are omitted in the drawing.

lVhen the circuit is set up as shown in Fig. 2, the line MLbeing out of service, the upper group of transmitting channels atst-ation W cooperate with the upper group of receiving channels at station E and the lower group of transmitting channels at station E cooperate with the lower group of receiving channels at station W to form a two-way carrier current telephone system employing the lifie ML as a transmission medium. -Voice-modu1ated waves originating in channel TC5, for example, are transmitted through transmitting amplifiers TA-Q, high pass grouping filter HP, jack 5, cord 12, jack 6', linc'ML,, rep alter filter HOE amplifier K -2, the direction of which has been reversed. filter I-IIF, line ML jack 9', cord 13, jack 8, high pass grouping filter HP, receiving amplifier RA2, receiving channel RC5, wherein the received wave is demodulated and the voice frequency component; is passed to the associated low frequency line in the usual manner. For transmission from station E to station \V, a voice-modulated wave originating for example in channel '1 l,1. is transmitted through transmitting amplifier Til -1, low pass filter LP line ML repeater filter LIF amplifier A,1, filter LOF line ML low pass filter LP receiving amplifier RA,1, receiving channel RC,1, wherein the received wave is demodulatcd and the voice frequency component is passed to th'e'associated low frequency line in the usual manner.

If, on the other hand, the line LIL, is disrupted and only the line ML remains in service, then the lower group of transmitting channels at station W cooperates with the lower group of receiving channels at station E and the upper group of transmitting channels at station E cooperates with the other group of receiving channels at station WV to .form a tw o\vay transmission system, utilizing the single transmission line ML. In this case, the higher group of frequencies transmitted from station E to station W are transmitted through amplifier A'2 of repeater R, the direction of which has been reversed in the mannerdescribed, while the lower group of frequencies transmitted from station W to station E are transmitted through repeater amplifier A1 the direction of which is unchanged. c

When the transmission line which has been rendered inoperative is again out back in service the system may be restored to its original condition by the simple manual operation of adjusting the terminal and repeater patching cords. In this way the system may be switched over from a fourwire circuit to operate half of the total number of channels on a two-wire basis with a minimum delay and maximum efllcicncy whenever occasion demands.

' Fig. 3 shows a modification of the invention in which a pilot current is utilized to automatically convert a four-wire system into two two-wire systems upon the failure of-either pair of line wires.

Since carrier current telephone systems usually provide forpilot channels for regulating the transmission line, the system of automatic control hereinafter described may readily be applied to carrier systems already in existence without otherwise changing the terminal or repeater circuits thereof.

In accordance with the invention, a pilot current of distinctive frequency is impressed i upon one pair of transmission wiresat one pair of transmission wires for transmission back to the first station.

Switching circuits under the control of the pilot current are provided at the terminalstations, as well as at intermediate repeater points, for automatically converting the four-wire system into two two-wire systems and for cutting off the transmission a of the pilotcurrent if either pair of line wires should fail. As soon as the disrupted line has been repaired and cut back in service, a second pilot current of distinctive frequency is transmitted over a circuit including both pairs of line wires and serves to operate mechanisms for again impressing the original'pilot current upon the transmission lines. The original pilot current,-

when transmitted over the circuit, serves to operate switching mechanisms to return the system to its normal four-wire operation.

' In adition to the signaling channels indicated in Fig. 3, which areidentical to those shown iii Figs. -1 and 2, a PC'1 is also associated with the line ML at station-WV at a point intermediate the lower group of transmitting channels and the transmitting amplifier TA-1.- The pilot channel PC-l includes the usual transmitting oscillator, amplifier and band filter. The pilot oscilator is adapted to continuously supply a pilot current of a frequency different from that of any of the signaling channels, but which is nevertheless within the transmission range of the low pass ter- ,-minal filters LP and the low pass repeater filters.

In operation, the pilot current originating in the pilot channel PC-1 is transmitted through the left and right hand contacts of relay 20, transmitting amplifier TA--1, low

pass filter L transmission line ML to the repeater R. At the repeater the pilot current is transmitted through low pass input filter LIF to the amplifier A--1 and then divides, a portion of the current being transmitted through low pass output filter LOF to the line ML and a portion of the current being transmitted through the 'pilot receiving circuit RP1. The pilot receiving circuit RP-1 comprises a band filter adaptedto selectively transmit the pilot current, and

an amplifier-rectifier which may be of the well known thermionic electron discharge type. The pilot current transmitted through this circuit is rectified and caused to operate a relay 21 in the output thereof 1 for a purpose hereinafter to be described.

The pilot current receivedatstation E from line ML again divides, a portion being transpilot channel" mittedover line MIL back to the station W through low pass input filter LIF,, amplifier A,1 and thereupondivides, a portion of the current being transmitted through the low pass output filter LOF to the line ML and a portion being transmitted to the receiving pilot circuit RP-3, which is identical to the pilot receiving circuits RP-1 and RP2. The relay 23 is maintained energized by the rectified current in the output of pilot receiving circuit- RP-3. The pilot current received from the line ML at station W is transmitted through low pass filter LP and is selectively transmitted by the pilot receiving circuit BP4,"where it is rectified and caused to energize the relay 24.

It will thus be seen that when the transmission lincs ML and ML are both in service the pilot current originating in the pilot channel PC1 is continuously transmitted ovenboth these lines, and during such transmission is caused to maintain relays 21, 22, 23 and 24 energized in the manner shown.

The systemwhen in this condition tune-- tions as a four-wire system, the transmission line ML being utilized for transmission from station W to station E and the transmission line ML, being utilized for transmission from station E to station W.

Therefore, voice-modulated waves originating in one of the lower group of trans-' mitting channels at station W, for example in transmitting channel TC1, are transmitted through transmitting amplifier selectively transmitted TA 1, low pass filter LP line ML, low pass repeater input-filter. LI amplifier A-1, low pass repeater output filter LOF, line ML, low pass filter LP at station E, receivlug-amplifier RA-1 to receiving channel R 1, where the received waves are demodulated and the voice frequency components are assed to the associated low frequency'telep one line in the usual manner. Voice-modulated waves originating in one of the upper groups of transmitting channels at station W, for example in channel TC-5, are transmitted through transmitting amplifier TA--2, high pass filter HP, armatures and left hand front contacts of relay 25, transmission line ML, high pass input repeater filter HIF, left hand armatures and back contacts of'relay 25, line ML, high pass repeater input'filter HIF, right hand back' contacts and armatures of relay 26, amplifier A2, hi h pass repeater output filter HOF, line ME, right hand'back contacts and armatures of relay 27, high pass filter HP, receiving amplifier RA2 to receiving channel RC5, where the received waves aredemodulated and the voice frequencycomponents are passed to the associated low frequency telephone line.

input filter LIF amplifier A,1', low pass repeater output filter LOF line ML low pass ouping filted LP receiving amplifier -A-,-1, receiving channel RC,1

. whence the demodulated waves are trans;

mitted to the associated low frequency tele-.

phone line. Voice modulated waves origi:

-nating in one of the upper group of trans- -mitting channels at station E, for example in channel TC,5, are transmitted through transmitting amplifier TA,2, high pass grouping filter HP,, left hand armatures and back contacts of relay 27, line MlL high pass repeater input filter'HIF,, right hand armatures and back contacts of relay 28, am-

plifier A,-2, left hand back contacts and,

armatures of relay 28, high pass repeater output filter HOF I line ML right hand back-contacts and a'rmatures of relay 25,

high fpass grouping filter HP receiving ampli er RA,'2 to receiving channel RC 5, whence'the demodulated'waves aretransmitted to the associated low frequency tele hone line. Y

I during the operation of the four-wire system as described above, either of the transmission lines ML or ML should be transmission to the disrupted or otherwise lost to" service, the

pilot circuit will motion to automatically convert the remaining transmission line .into a two-.wa path for the transmission of signals in bot directions between terminal stations. Assume, for example, that the transmission line has been disrupted at apoint' 7 between station W and repeater R, thereby.

preventing the transmission of signals over this line. In this case, the pilot current originating in channel PC1 is cut off from pilot receiving circuits RP 1, RP-2, RP-3 and RP-4. Since rectified current no longer flows in the output of these pilots receiving circuits the relays 21- 22, 23 -and 2a are deenergized.

Y 1 elay 21 in releasing its armature estab lishes an operating circuit for relay 28, which extends from ground through battery 29, armature and contact of relay 21, winding of relay 28 to ground. Relay 22 in releasing its armature establishes an operating circuit for relay 27 which extends from ground, through battery 30, armature and contact of relay 22, winding of relay 27 to ground. Similarly, the deenergization of relay 23 causes the operation of relay 26 over a circuit extending from ground through battery 31, armature and contact of relay 23, winding of relay 26 to ground. Relay 24 in releasing its armature establishes an operating circuit for relay 25, which may be traced from ground through battery 32, armature and contact of relay 24, winding of relay 25 to ground, and also operates relay 20 over a circuit extending from ground through battery 32, armature and contact of relay 24, winding of relay 20 to ground.

The energization of relays 25, 26, 27 and 28 accomplished in the manner described above causes each of these relays to attract both left and right hand windin in each case opening the backcontaets 0 these relays which are shown elosedin. the drawing'and closingl the respective front contacts thereof. The operation of relay 25 disconnects the upperegroup of transmitting channels, TC.5, etc., at station W from th'e disruptedline ML and connects these channels to the line ML,, and at the same time disconnects the upper group of receiving channels 110 -5, etc., from line ML and connects these-channels to the line ML. The lower group of transmitting and receiving channels at station W are unaffected by this switching operation. At station W, therefore, the lower group of receiving channels RC,-1, etc., and the upper group of transmitting channels TC5, etc., are associated with the line ML which is still in service.

The operation of relay 27 at station Edisconnects the upper group of receiving channels RC5, etc., from the disrupted line ML and connects these channels to the line ML,, and at the same time disconnects the upper group of transmitting channels TC 5, etc., om line ML- and connects these channels to line LIL. The lower group of transmitting and receiving channels at station E are not affected by this switching operation. Ac-

eordingly, the upper roup of receiving channels RC5, etc., an the lower group of transmitting channels TC 1, etc., are now connected in the transmission line ML and are ada'ted to cooperate with the upper group 0 transmitting channels and the lower group of receiving channels at station W for the two-way transmission of signals over line ML y The operation of relay 28 at repeater R, serves to reverse the direction of repeater amplifier A, 2, whereby the upper group of frequencies transmitted from station may be transmitted through am lifier A,-2

to station E.' The operation 0 relay 26 at repeater R reverses the repeater amplifier The energization of relay 20 at station W causes this'relay to attract its armatures to disconnect the line ML.

lVhen the system has thus been setup to function as a two-wire system using the line ML as a two-way transmission path, voice ting channel TC-5 at station W are transmitted through transmitting amplifier TA2, high pass grouping filter HP, left hand armatures and'front contacts of relay 7 left hand armatures and front. contacts of reinstead of occurring at a point intermediate 25, line ML high pass repeater filter HOFQ,

station E,high pass grouping filter HP, rc-

ceiving amplifier-,RA-2,to receiving channel RO-5, whence the demodulated waves are transmitted to the associated low frequency telephone line.

If the break in the transmlssionline ML station \V and repeater R had occurred be tween repeater R and station E the operation of the switching circuits would'nevertheless be the same as that described above. In this case, although theilot current originating in pilot channel P 1 would momentarily be transmitted to the pilot; receiving circuit RP thus maintaining relay 21 energized, nevertheless since the-pilot current is cut oil from pilot receiving circuit RP-4 and re,- lay 24 is accordingly deenergized the relay 20 is immediately operated to disconnect the pilot channel PC'1 from the line 'ML, whereupon the'relay 21 at repeater R is deenergized thus causing the operation of re-' lay 28 at repeater R,. i:

When the line ML has been prepared and is cut back in service the system is automatically returned to its original conditionzto 'function as a four-wire s stem. This is accomplished 1n the following manner:

A pilot channel PC-2 is connected to line ML at station W and'is ada ted to furnish a pilot current of distinctive requencyw'ithpilot channel PC--,1 froml the pilot current generated thereby is normally suppressed from transmission over the line ML due to the fact that when the system is operated on a four-wire basis this pilot current cannot be transmitted through the unilateral amplifier A -2 of repeater R However, as soon as the system is converted to operate on atwo-wire basis as described above the pilot current from channel PC-2 will readily be transmitted through repeater amplifier A,-2. Therefore when the disrupted line ML is cut back in service the pilot current from channel PC2 is transmitted through line ML,, high pass repeater filter HOF left hand armatures and front contacts of relay 28, amplifier A 2, right, hand front contacts and armatures of relay 28, high pass repeater filter HIE, line ML pilot amplifier PA2, pilot filter PF2, line ML, high pass repeater filter HOF, right handarmatures-and front contacts of relay 26 since relay 26 is energized, amplifier A'2, left hand front contacts and armatures of relay 26, high passrepeater filte r HIF, line ML to the pilot-receiving circuit- RP-5. The pilot receiving circuit RP,5 is similar in construction to pilot receiving circuits RP' 1,' RP- 2 etc., andthe rectified currents in the output of this circuit'are caused to operate 'pilot "relay 33. The operation of relay 33 serves to connect pilot channel PC1 to'the common transmitting circuit intermediatethe lower groupoftransmitting channels TC-l, etc., and transmitting amplifier TA1.

'24 to restore the'system to the original condition shown in the drawing.

The pilot current from channel PC-2 is thus cut off from transmission since it cannot pass through the unilateral amplifier' A 52 of repeater R and relay 33 in the output of the pilot receiving circuit RP-5 is deenergized. -However,.relay 24' at station 'in attracting its armature deenergizes. relay 20, the armatures of which, in turn, fall back and establish a circuit from pilot channel PC- -l to the line ML. The pilot current in channel PC-lis thus again continuously transmitted over the system and adapted to cause the switching operations The pilot. current from channel PC-l is thus again described above should either of the lines ML'or ML -be again out out ofservicc; v

-When the system is converted automatically in the manner described above, the low frequency lines at the terminal stations may also be shifted to correspond to the altered condition of the high frequency circuits. The reversing mechanisms for the low frequency circuits may be controlled by the pilot circuits in the same manner as that described in connection with quency circuits.

When the line lVIL is disrupted while line ML remains in service the systemis set up to operate on a two-wire basis in the same manner as that described above except that in such case the transmissions are carried on in both directions over the line ML..

The invention set forth herein is, of course, susceptible of various modifications and adaptations, and accordingly the invention is. not torbe considered as limited to the particular disclosure.

What is claimed is: A

1. Amultiplex signaling system comprising two terminals,a pair of-transmission lines and a repeating station for' connecting said terminals, meansat eachv of said terminals for transmitting signals in one direction only the high fre- .over one of 'said lines and for receiving signalsm the opposite direction only over the other of said lines,,switching means at each of said terminals 'for interchanging'a portion of said transmitting and receiving means and switching means for interchanging a portion of the apparatus at said retively. comprising in two diflerent frequency I ranges over one of said lines and for receivmg waves in two groups of frequencies respectively comprised in said two frequency ranges over the other of said lines, and

switching means at each of said terminals for connecting the receiving means of one of said frequency groups with the transmitting line and for/connecting the transmitting main-s of one of. said frequency groups with the receiving line thereat, whereb signals ateach station may be transmitted in one direction by waves within one of said frequency ranges and received by waves within the other of said frequency ranges over one of said line is disrupted.

3. A multiplex signaling system comprising two terminals, a pair of transmission lines connecting said terminals, two groups of transmitting and receiving channels at each of said terminals for transmitting carrier waves "within a high and a low frequency range in one direction over one of said lines and for receiving carrier waves within said high and saidlow frequency range inthe opposite direction over the other 0f said lines, switchingmeans at each of said terminals for connecting one of the groups ofreceiving. channels to the trans- .direction of amplification on one 0 lines when the other;

mitting line thereat and switching means at each of said terminals for connecting one of the groups of transmitting channels to the receiving line thereat, whereby signals at each station maybe transmitted in one direction by waves within one of said frequency ranges and received by waves within the other of said frequency ranges over one of said lines when the other line is disrupted.

4. A multiplex signaling system comprising two terminals, a pair of transmission lines connectingsaid terminals, two groups of transmitti ng, and receiving channels at each of said terminals for transmitting carrier waves in one direction'over. one of said lines and for receiving carrier waves in the opposite direction over the other of said lines, a repeater comprising two channels connected to each of said lines at a point 4 the waves comprised in the respective groups in one direction, switching means at each of said terminals for connecting one of the groups of receiving channels to the transmitting line thcrcat, switching means at each of said terminals for connecting one of the groups of transmitting channels to the receiving line thereat, and switching means at each of said repeaters for reversin the the repeater channels, whereby waves may be transmitted in both directions over one of said lines when the other line is lost to service. A

5. A multiplex signaling system comprising a plurality of transmission lines, means for insuring the transmitting of signals in one direction only over each of said lines under normal operating conditions, and means for varying the connections of said lines automatically to transmit signals in the opposite direction over one of said lines when another of said lines is disrupted.

6. A multiplex signaling system comprising two terminals, a pair of transmission lines connecting said terminals, means at said terminals for insuring the transmitting of signals in one direction only over each of said lines, and automatic switching means operated under predetermined conditions for varying the connections of said lines to transmit signals in the other direction over one of said lines.

7. A multiplex signaling system comprising two terminals, a pair of transmission lines connecting said terminals, means at each of said terminals for transmitting waves comprised in two grou s of frequenceiving means of one of said frequency groups with the transmitting line and for lines and switching means operated by said control current under predetermined conditions for transmitting signals in the other direction over one of said l1nes.

lines connecting said terminals, two groups of transmitting and receiving channels at each of said terminals for transmitting carrier waves in one direction over one of said lines and for receiving carrier waves in the opposite direction over the other of said lines, means for transmitting a control current over said lines, and switching means operated by said control current under pre determined conditions for connecting one of the groups of receiving channels to the transmitting line and for connecting one of the groups of transmitting channels to the receiving line thereat.

prising two terminals, a pair of transmission lines connecting said terminals, two groups of transmitting and receiving channels at each of said terminals'for transm1t ting carrier waves in one direction over one of said lines and for receiving carrier waves in the opposite direction over the other of said lines, a repeater comprising two channels connected to each of said lines intermediate said terminals for amplifying the waves comprised inthe respectivegroups' in the one direction, means for transmitting a pilot current over a loop circuit includlng both of said transmission lines, switching means at each of said terminals operated by the failure of said pilot current to connect one of the groups of receiving channels to the transmitting line and to connect one of the groups of transmitting channels to the receiving line thereat, and switching meansf operated by the failure of said pilot current at each of said repeaters for reversing the directionof-amplification of one of the repeater channels, whereby waves maybe transmitted in both directions'over one of said lines when the other line is disrupted.

11. A multiplex signaling system comprising two terminals, a pair of transmission lines connecting said terminals, two groups of transmitting and receiving channels at each of said terminals for transmittin carrier waves in one direction over one 0? said lines and for receiving carrier waves in the opposite direction over the other ofv said lines, a repeater comprising two channels connected to each of said lines intermediate said terminals for amplifying the waves 9. A multiplex signaling system compns mg two termmals, a pair of transmission comprised inthe respective groups in the one direction, means for transmitting a pilot current over a loop circuit meluding'both of said transmission lines, switching means at whereby Waves may be transmitted in both 10. A multiplex signaling system comdirections over one of said lines when the other line is disrupted, means controlled by said terminal and repeater switching means for transmitting a second ilot current over said transmission lines un er predetermined conditions, and switching means controlled by saidsecondpilot current for again impressing said-first pilot current upon said transmission lines. v

12. In a carrier signaling system, a pair of one-way lines, carrier transmitting and carrier receiving sets, means for connecting said sets to saidone-way lines to form a four-wire two-way system using the same carrier frequencies for transmission'in both directions and means for altering the connections of certain of said sets to said lines to convert either line into a two-way carrier signaling lineemploying 'difi'erent carrier frequencles for transmission in opposite directions.

In witness whereof, I hereunto subscribe my name this 18th day of September, A. D 

